Event-related coherence and event-related desynchronization/synchronization in the 10 Hz and 20 Hz EEG during self-paced movements

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Abstract

To investigate the activity of cortical regions in the control of movement, we studied event-related desynchronization/synchronization (ERD/ERS), event-related coherence (ERC), and phase coherence in 29-channel EEGs from 9 subjects performing self-paced movements of the right index finger. Movement preparation and execution produced ERD over the sensorimotor areas at 10 Hz and 20 Hz, followed by ERS. ERD corresponded spatiotemporally to an increase in coherence over the frontocentral areas. For both frequency bands, ERD began over the left sensorimotor areas and became bilateral at the time of movement onset. The coherence increase with frontal areas began in the left central areas and became symmetrical after EMG onset. The ERD and coherence increase was longer at 10 Hz than at 20 Hz. Phase coherence at 10 Hz showed a lead of anterior regions to posterior regions throughout the time period, and at 20 Hz showed a tendency toward zero phase delay corresponding with the movement. EEG desynchronization parallels functional coupling over sensorimotor and frontal areas. Event-related coherence and phase coherence findings implicate the frontal lobes in control of movement planning and execution. The involvement of different frequency bands with different timings may represent parallel changes in the cortical network.

Introduction

Cortical activation related to movement preparation and execution has been shown to desynchronize the mu rhythm (Chatrian et al., 1959) in a phenomenon called event-related desynchronization (ERD) (Pfurtscheller and Aranibar, 1989). ERD in hand movement is more prominent over the contralateral sensorimotor areas during motor preparation and extends bilaterally after movement initiation (Chatrian et al., 1959; Pfurtscheller and Berghold, 1989; Defevbre et al., 1993; Dujardin et al., 1993). During movement preparation and execution, event-related synchronization (ERS) in the 10 Hz band occurs during movement preparation and execution over areas not involved in the task (Pfurtscheller, 1991; Pfurtscheller, 1992; Pfurtscheller and Neuper, 1994) or, after movement execution, over the same areas that previously displayed ERD (Pfurtscheller, 1981; Pfurtscheller, 1992; Toro et al., 1994). Some investigators (Pfurtscheller, 1992; Pfurtscheller and Neuper, 1994) have hypothesized that the 10 Hz ERD is an indicator of cortical activation and that the 10 Hz ERS is expressed by cortical areas in an idling state. Disparate findings have been reported for frequencies higher than the alpha band (Pfurtscheller, 1981; Pfurtscheller and Neuper, 1991; Arroyo et al., 1993; Pfurtscheller et al., 1993; Kristeva-Feige et al., 1993; Nashmi et al., 1994; Pfurtscheller et al., 1994; Toro et al., 1994). These studies differed in the task, frequencies studied, and temporal resolution of the analysis.

Coherence analysis is a useful indicator of functional connections between different cortical areas (Sklar et al., 1972; Busk and Galbraith, 1975; Thatcher et al., 1986). Coherence between two EEG signals is the spectral cross-correlation normalized by their power spectra. When a coherent frequency band is present in two signals, phase coherence can indicate whether the oscillations are occurring synchronously or if one signal is leading the other (Enochson and Otnes, 1968; Jenkins and Watts, 1968). Since coherence is normalized by the power of a given frequency band, it is independent of the amplitude of the oscillations in the two signals. This property guarantees the usefulness of coherence analysis during dynamic power changes, such as ERD/ERS. In a study of visual and auditory cued left or right finger movements, Rappelsberger et al. (1994)reported that during the 10 Hz ERD over the sensorimotor areas, the 10 Hz coherence increased between the central and the frontal electrodes and decreased between the central and the temporal electrodes. Only local and interhemispheric coherence changes at the 10 Hz frequency were analyzed. Thatcher (1995), using a different algorithm (pseudoinverse-derived dipole time series), showed complex evolution of instantaneous coherence between homologous sensorimotor areas and between the sensorimotor areas contralateral to the movement at the midline central areas during a time span of about 1 s during a self-paced finger movement. The time course analyzed was in the theta frequency band (4–7 Hz) and no ERD/ERS correlates were described. With the goal of investigating the role of different brain regions in the control of movement, we investigated the spatiotemporal dynamics of cortical functional connections and the relationship between coherence and ERD/ERS in self-paced movements.

Section snippets

Methods

We studied 9 right-handed normal subjects (6 women and 3 men), aged 19–36 years. The protocol was approved by the Institutional Review Board, and all subjects gave their written informed consent for the study.

Results

The percentage power changes within the 10 Hz and 18–22 Hz frequency bands are shown in Fig. 1A,B. At both frequency bands, ERD started at −1.75 s over the left sensorimotor areas (central and, to a lesser degree, parietal) and became bilateral after movement onset. The 10 Hz ERD was relatively long-lasting and replaced by ERS only 2 s after EMG onset. The 18–22 Hz ERS, however, followed ERD as early as 0.75 s from EMG onset. Some topographic differences between 10 Hz and 18–22 Hz ERD/ERS were

Discussion

This paper describes the relationship between ERD/ERS and event-related coherence in a self-paced movement. Rappelsberger et al. (1994)used a visual and auditory cued task corresponding to a contingent negative variation (CNV) paradigm. In that study, the EEG was prefiltered with narrow band-pass centered at 10 Hz, and FFT was performed on epochs of 125 ms embedded in time windows of 2 s with zero padding. Only local coherence between adjacent electrodes in the same hemisphere and

Acknowledgements

We thank Ms. Joan Trettau, R. EEGT., for technical assistance, and D.G. Schoenberg, M.S., for skillful editing.

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